With the increasing popularity and demand for portable electronic devices for computing, communication, and entertainment, the need for batteries for use in such portable applications has also increased. There is a particular need for rechargeable, i.e., secondary batteries, in portable device applications. Batteries that are reliable, long-lasting, low-cost, and environmentally friendly, yet which possess both high energy and power densities are most desirable. There is a particular need for high energy density secondary batteries for use in portable military applications. Batteries that exhibit energy and power densities of up to about 110 Wh/kg and 40 W/kg, respectively, at an operating current density of about 20 mA/cm2 are preferred for military applications.
In theory, alkali metal batteries, most importantly those where the alkali metal is lithium, utilizing an alkali metal anode, an alkali metal ion-conducting polymer electrolyte and an alkali metal-intercalating cathode, can provide secondary batteries for portable applications and meet the preferred performance characteristics for portable military applications. However, the application of polymer electrolytes in electrochemical cells, particularly in battery construction, has been restricted by inadequate ionic conductivity of the electrolyte. Most materials that have been examined possess values between 10xe2x88x929 to 10xe2x88x925 S/cm at room temperature. Target ionic conductivities to meet preferred performance characteristics under ambient conditions, are in the 10xe2x88x923 to 10xe2x88x922 S/cm range. The most widely studied material, poly(ethylene oxide) (PEO), incorporating lithium salts such as LiClO4 and LiCF3SO3, demonstrated ionic conductivities well below the 10xe2x88x923 S/cm target at room temperature (Berthier, C. et al. (1983) Solid State Ionics 11:91; Shi, J. and Vincent, C. A. (1993) Solid State Ionics 60:11; Chang, W. and Xu, G. (1993) J. Chem. Phys. 99:2001; Torell, L. M. et al. (1993) Polym. Advan. Technol. 4:152).
Polymer blends and copolymers of various linear and cross-linked polymers have been examined as polymer electrolytes (Li, N. et al. (1992) J. Appl. Electrochem. 22:512; Prabhu, P. V. S. et al. (1993) J. Appl. Electrochem. 23:151; Takeoka, H. A. and Tsuchida, E. (1993) Polym. Advan. Technol. 4:53), including poly(vinyl acetate) (Greenbaum, S. G. et al. (1985) Solid State Ionics 18-19:326), poly(dimethyl siloxane) (PDMS) based matrices (Greenbaum, S. G. et al. (1986) J. Appl. Phys. 60:1342), propylene carbonate or ethyl carbonate (Abraham, K. M. and Alamgir, M. (1990) J. Electrochem. Soc. 137:1657; Alamgir, M. et al. (1991) in xe2x80x9cPrimary and Secondary Lithium Batteries,xe2x80x9d K. M. Abraham and M. Solomon (eds.), Electrochem. Soc. Proc. Ser. PV91-3:131; Alamgir, M. and Abraham, K. M. (1993) J. Electrochem. Soc. 140:L96; Huq, R. et al. (1991) in xe2x80x9cPrimary and Secondary Lithium Batteries,xe2x80x9d K. M. Abraham and M. Solomon (eds.), Electro-chem. Soc. Proc. Ser. PVxe2x80x9491-3:142; Huq, R. et al. (1992) Solid State Ionics 57:277; Huq, R. et al. (1992) Electrochim Acta 37:1681), poly(propylene oxide) (Greenbaum, G. et al. (1988) Brit. Polym. J. 20:195), and poly[bis(methoxyethoxy) ethoxy phosphazene] (MEEP) (Greenbaum, S. G. et al. (1991) Mat. Res. Soc. Symp. Proc. 210:237). Although some incremental ionic conductivity performance improvements were realized with these materials, ionic conductivities of 10xe2x88x923 S/cm at room temperature were not achieved.
Dielectric properties and ionic conductivities of lithium triflate complexes of polysiloxanes (average molecular weight 4500-5000) having certain cyclic carbonate side chains have also been examined (Z. Zhu et al. (1994) Macromolecules 27:4076-4079). These derivatized polysiloxanes were reported to be very viscous and to exhibit maximum lithium ion conductivities of 1-2xc3x9710xe2x88x924 S/cm.
Desirable features in technically useful polymer electrolytes include: i) high ionic conductivity at or close to ambient temperatures; ii) ionic transport numbers of unity or near unity for the cation of interest; iii) the ability to maintain mechanical integrity and dimensional stability within a cell subjected to electrochemical cycling; iv) environmental stability; v) the ability to maintain stable interfacial regions between electrodes; and vi) safety. There remains a significant need in the art for polymer electrolytes conductivity and mechanical properties suitable for battery applications and particularly for use as interelectrode spacers in such batteries.
This invention provides alkali ion-conducting polymer electrolytes having high ionic conductivity and improved elastomeric properties compared to currently available materials. These polymer electrolytes are useful, for example, in high energy density secondary batteries for portable electronic devices.
The polymer electrolytes of this invention consist of polymer matrices complexed with alkali metal salts. The ability of polymers, most notably polyethers, to chelate alkali metal cations is used to achieve ionic conduction within these materials. The electrolyte is formed by solubilizing an alkali metal salt in a polymer matrix which facilitates ionic dissociation and enhanced ion mobilities. The polymer electrolytes of most interest are those incorporating lithium ion salts and which exhibit high lithium ion conductivity at or below ambient temperatures. The cross-linked siloxane polymer electrolytes of this invention also possess favorable elastomeric properties for use as thin and flexible interelectrode layers for construction of battery cells and batteries.
Provided herein is an alkali ion-conducting polymer electrolyte comprising a cyclic carbonate-containing polysiloxane preferably treated with a modification agent capable of crosslinking the siloxane or extending the chain length of the siloxane, and having an alkali metal ion-containing material solubilized therein. Preferably the alkali metal ion is lithium.
The alkali ion-conducting polymer electrolytes of this invention comprise a polysiloxane derivatized with cyclic carbonate groups and preferably treated with crosslinking agents and/or polymer chain extenders (modification agents). The carbonate groups facilitate ionic dissociation and treatment with crosslinking/chain extension agents is believed to provide desirable elastomeric properties. Polymer electrolytes are prepared by treatment of a cyclic carbonate-containing polysiloxane with a crosslinking agent or a polymer chain extension agent (modification agent) in the presence of an alkali metal ion salt, preferably a lithium salt. This strategy exploits the concept that carbonate oxygens, within a single phase carbonate-siloxane polymer matrix, facilitates extensive ionic dissociation of introduced alkali metal salts, and that furthermore elastomeric behavior of the matrix under ambient temperature conditions leads to enhanced mobility of lithium ions. Polymer electrolytes having these properties permit small interelectrode distances to be achieved within portable secondary lithium batteries.
Preferred polymer electrolytes of this invention exhibit alkali metal ion conductivities in the range of 10xe2x88x924 to 10xe2x88x922 S/cm or higher. Preferred polymer electrolytes having these properties that are useful for applications in batteries exhibit glass transition temperatures that are lower than ambient temperature. In particular, the use of ionically conducting polymeric electrolytes facilitates the fabrication of thin-layer, flexible battery designs provided that the polymer can maintain a reliable interelectrode spacing without electronic shorting. This ability facilitates achieving low internal resistance and thereby improving electrochemical performance in terms of delivered energy density and discharge performance.
More specifically, polymer electrolytes of this invention are prepared by crosslinking or chain extension (modification) of internally derivatized polysiloxanes (I) or end dervatized polysiloxanes (II) comprising at least one derivatized Si which can be represented by: 
where M can be R, Rxe2x80x2 or a linked cyclic carbonate group: 
where
n and m are integers where n+m is preferably 10 or less, X is O, S, CO, OCO, or COO, x is 0 or 1, p is 1 or 2, q is a positive integer preferably 1 to about 100, r+s=q, Y is a linking group which can be a xe2x80x94CH2xe2x80x94 chain, a halogenated xe2x80x94CH2xe2x80x94 chain, or a xe2x80x94CH2xe2x80x94 chain or a halogenated xe2x80x94CH2xe2x80x94 chain which contains one or more O, S, CO, COO, or OCO, (e.g., ethers, thiocther, esters, etc.) group wherein the xe2x80x94CH2xe2x80x94 chain preferably contains less than about 10 carbon atoms and at least one of M is a linked cyclic carbonate group; and
R and Rxe2x80x2, independent of other R and Rxe2x80x2, can be a hydrogen, hydroxy, an alkyl alkenyl, an alkoxy, an hydroxyalkyl (e.g., xe2x80x94(CH2)nxe2x80x94OH, where n=1 to about 20, preferably 1 to about 6), halogenated alkyl or halogenated alkenyl group, preferably having 6 or fewer carbon atoms.
R and Rxe2x80x2 groups on the same Si atom may be the same or different groups.
The cyclic carbonate group can be covalently linked to the polysiloxane backbone using a variety of linking groups. Preferred linking groups are hydrocarbons, ethers, thioethers, esters, and ketones. More preferred linking groups are hydrocarbons and ethers. The linking groups can be halogenated, e.g., with F, Cl or Br. The cyclic carbonate can have a five- or six-member ring.
More specifically, internally derivatized polysiloxanes or end derivatized polysiloxanes include compounds of formulas: 
where variables are as defined above.
Carbonate derivatized polysiloxanes are crosslinked or chain-extended in the presence of alkali metal ions to obtain improved polymer electrolytes. Modifying agents are preferably silanes carrying alkoxy, alkene, and/or acyl groups, and preferably carrying ethoxy, vinyl or acetoxy groups. Polysiloxanes, such as polymethylhydrosilane, can be internally derivatized and polysiloxanes, such as silanol-terminated polydimethylsiloxane or vinyldimethyl-terminated polydimethylsiloxane, can be end-derivatized with cyclic carbonate side-chains. Preferred electrolyte polymers are crosslinked poly(alkylhydrosiloxanes) internally derivatized with cyclic carbonates. More preferred electrolyte polymers are crosslinked poly(methylhydrosiloxanes). Preferred crosslinking agents are methyltrimethoxysilane, methyltriethoxysilane, methyltriacetoxysilane, tetramethoxysilane, tetraethoxysilane, tetraacetoxysilane, vinylmethyldiethoxysilane, vinylmethyldiacetoxysilane, and mixtures thereof. More preferred crosslinking agents are methyltriacetoxysilane, tetraethoxysilane, vinylmethyldiethoxysilane, vinylmethyldiacetoxysilane, and mixtures thereof. Preferred starting polysiloxanes, i.e., prior to crosslinking, have average molecular weights ranging from about 400 to about 5000 (preferably about 1500 to about 5000) and include among others: polymethylhydrosiloxane; polydimethylsiloxane (silanol terminated); and polydimethylsiloxane (vinyl dimethyl terminated).
The starting polysiloxane is derivatized with one or more carbonate side chains with preferred internally derivatized polysiloxanes carrying from on average 0.5 to about 10 carbonate side-chains/polysiloxane. More preferred internally derivatized polysiloxanes carry on average 1 or 2 cyclic carbonate side-chain. End-derivatized polysiloxanes preferably carry on average 1 or 2 carbonate side-chains.
Polysiloxanes with cyclic bearing carbonate side chains are reacted with various crosslinkers or chain extenders to enhance the stability or elasticity of the parent polymer.
The electrolyte polymer of this invention can be prepared from a mixture of derivatized polysiloxanes having different average molecular weights, different substituents, and different cyclic carbonate side-chains and can be crosslinked or chain-extended with a mixture of crosslinking and/or chain-extending agents. Electrolyte polymers of this invention have a glass transition temperature (Tg) below ambient temperatures, (room temperature, orxcx9c20xc2x0 C.-25xc2x0 C.), i.e., about 15xc2x0 C. to about xe2x88x92100xc2x0 C. Preferred electrolyte polymers are those that exhibit Tg below about xe2x88x9220xc2x0 C. and more preferably below about xe2x88x9240xc2x0 C.
Polymer electrolytes include solubilized alkali metal salts. Preferred salts are lithium salts, more particularly LiClO4, LiAsF6, and LiCF3SO3. Alkali metal salts are solublized in the polymer electrolyte to provide a homogeneous material. Polymer electrolytes incorporate alkali metal ions in a molar ratio of about 1:30 to about 1:5 alkali metal ions to cyclic carbonate groups in the polysiloxane. Preferred polymer electrolytes incorporate alkali metal ions in a molar ratio of about 1:10 to about 1:20 to cyclic carbonate groups in the polysiloxane.
The polymer electrolyte is prepared by initial reaction of the derivatized polysiloxane, modification agent, alkali metal salt and catalyst in an appropriate solvent system which is preferably selected to provide a homogeneous reaction solution. After completion of reaction, as indicated by either the decrease in the Si-H bond as determined by FTIR for the derivatization of polysiloxane and cyclic carbonate, or by other methods known to one of ordinary skill in the art, the polymer electrolyte is molded or shaped into a desired form and cured. Modification agent is typically used in excess. The preferred ranges of polysiloxane:modification agent are dependent upon the reactive groups within the derivatized polysiloxane, but typically should be within 1.5:1-2:1 carbonate polysiloxane: crosslinker. Preferred solvent systems and catalysts are also dependent upon the materials used, but for a typical polymer, acetonitrile, acetone, tetrahydrofuran, benzene and mixtures thereof are preferred solvents. In general any known crosslinking or chain-extension catalyst can be employed. Platinum divinyl tetramethyldisiloxane acetic acid or titanium (IV) triethanolaminate isopropoxide are preferred catalysts.
This invention also provides batteries employing the ion-conducting polymer electrolytes described herein. These batteries comprise a first electrode comprising an alkali earth metal; a second electrode comprising one or more transition metals; and a separator comprising an alkali ion-conducting polymer electrolyte, wherein the separator is in physical contact with both the first electrode and the second electrode. Typically, the first electrode acts as an anode, and is preferably lithium. Typically, the second electrode acts as a cathode, and preferably comprises one of the following: TiS2, LiMn2O4, LiCoO2, LiNiO2, CuxAgyV2OzLi1.5Na0.5MnO2, LixMnO2, LiSO2 and V6O14. More preferably, the second electrode comprises V6O13 or LixMnO2. The second electrode can also contain an alkali ion-conducting polymer electrolyte as a component of the electrode. The second electrode can also comprise an organo-sulfur polymer, such as 2,5-dimercapto-1,3,4-thiadiazole on a substrate.
In preferred battery construction, a film or layer of polymer electrolyte is placed in contact with the second electrode (deposited on the interelectrode cathode surface), or introduced between the electrodes. Preferred battery cells comprise an anode and a cathode separated by a layer of polymer electrolyte. The polymer layer is preferably 0.05 to 0.15 mm thick, more preferably about 0.1 mm thick. Preferred batteries have an overall thickness of 1-4 mm. The layer of polymer electrolyte should be thick enough to avoid shorting, yet thin enough to be easily shaped or rolled to fit into a confined space and still be operable.
Other applications for the polymer electrolyte include rechargeable lithium batteries, communication devices and various other portable electronics that can utilize light weight, compact, high energy density batteries.